You love Kingdom Hearts and you insist the unnumbered titles are spin-offs because they aren't part of the main narrative. I'm sorry, but any KH fan anywhere will tell you BBS, DDD, and CoM are integral chapters of the story. And beyond that, there are fans who will argue the importance of Days and Coded. And what about Union X? It's literally setting the stage for KH4.
Your qualifiers for spin-off don't match the reality of the games you're talking about. It'd be one thing if you wanted to be obtusely conservative in what a spin-off is and just say "anything unnumbered is a spin-off". At least you would be logically consistent. But you literally have no ground to stand on based on your own definition of what a spin-off is to say Dream Drop Distance is one, the only ground you have to stand on with Birth By Sleep is you don't play the as Sora (despite other non-spin-off games in other series featuring new characters and the fact that Sora still plays an important role of Ventus' story), and the only argument you have for Chain of Memories is the combat (though a more hardcore KH fan would point out the heart of CoM's combat, that being stocking abilities with a cool down, seeped into the rest of the series and negates the qualifier). Even Days evolved the combat a bit while shedding light on a semi-important character in KH3 that played a huge role in Lea's character arc.
The only unnumbered title I would near-completely relinquish as a spin-off is ReCoded due to the fact that the story amounts to "And then Mickey remembered BBS happened", but even that evolved the combat a bit and set the stage of DDD.
Oh my god, you are actually trying to gatekeep Kingdom Hearts. Get over it, I love Kingdom Hearts and think differently than you about it.